This invention relates to polyimides, and more particularly, to polyimides which are the intercondensation product of mixed dianhydrides and aromatic diamines, to composites formed therefrom, and to a method of improving the glass transition temperature of polyimides formed from the intercondensation product of mixed dianhydrides.
Polyimides are well-known in the art and have been widely used in applications requiring high thermal stability and oxidative stability. Polyimides are generally obtained by reacting specific tetracarboxylic acids or dianhydrides with primary diamines to obtain polyamic acids which can subsequently be converted to the corresponding polyimide.
In U.S. Pat. No. 4,336,175 which is incorporated by reference herein in its entirety, stoichiometric monomer imbalances of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane (6FTA) and an aromatic diamine in solvent mixtures are used to provide a binder solution having an increased temperature range over which a laminate of this binder and reinforcing filler can be compacted.
In the prior art, meta- and para-phenylenediamine have been condensed with various tetracarboxylic acid dianhydrides, such as pyromellitic dianhydride (PMDA) or bis(3,4-dicarboxyphenyl)ether dianhydride, to form coalescable polyimide powders of high thermal stability as in U.S. Pat. No. 3,422,061 which is incorporated herein by reference in its entirety. Bis(3,4-dicarboxyphenyl)ether dianhydride is also known as 4,4'-oxydiphthalic anhydride (ODPA). In U.S. Pat. No. 3,422,061, the polyimides are prepared in the form of powders which are fabricable in the range of 300.degree. to 550.degree. C. Combinations of various aromatic tetracarboxylic dianhydrides, such as bis(phenyldicarboxylic anhydrides) and aromatic amines, such as bis(phenylamines) are also discussed in U.S. Pat. Nos. 3,234,181 and 3,424,718, both of which are incorporated herein by reference in their entirety. In an attempt to improve the glass transition temperature (to between about 220.degree. C. to about 385.degree. C.), polyimides were prepared by reacting 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) with at least one aromatic primary diamine in U.S. Pat. No. 3,959,350 which is incorporated by reference herein in its entirety.
The use of two dianhydrides with aromatic diamines has also been shown in the art in U. S. Pat. Nos. 3,407,176, 4,612,361, 4,680,373, 4,794,157 and 4,864,015, all of which are incorporated herein by reference in their entirety. In U.S. Pat. No. 4,864,015, the glass transition temperature (Tg) of aromatic polyimides was improved by intercondensing aromatic dianhydrides with one or more aromatic diamines, and by using thianthrene dianhydride and one or more aromatic diamines, the resulting polyimides had Tg's in excess of 400.degree. C. In U.S. Pat. No. 4,612,361, two dianhydrides, one of which is a fluoro-containing dianhydride, such as 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), and p-phenylenediamine are used to prepare polyetherimides. The polyetherimides of U.S. Pat. No. 4,612,361 exhibit both increased Tg and increased solubility in halogenated solvent, and although it is shown therein that the 6FDA raises the Tg of the polyetherimide, the Tg of the corresponding polyetherimide was raised from 217.degree. C. (without 6FDA) to only 235.degree. C. (with 6FDA) in a polyetherimide made from the mixed dianhydrides wherein the other dianhydride was bisphenol-A dianhydride (BPADA).
Polyimides based on 6FDA are commercially available, and polyimides based on ODPA have been discussed above. Polyetherimides are described in U.S. Pat. No. 4,794,157 where ODPA is used as the other dianhydride with BPADA or PMDA and aryl diamine. The polyimides made from 6FDA or ODPA and meta- or para-phenylenediamines have adequate thermal and oxidative stability to survive 100-500 hours exposure in air at 371.degree. C. with minimal degradation. The Tg of polyimides derived from 6FDA and phenylenediamine range from about 350.degree. C. to 371.degree. C., and the Tg of polyimides derived from ODPA and phenylenediamine is about 300.degree. C. However, the Tg of these prior art polyimides does not permit their use in structural applications at 371.degree. C. It is generally desirable to have the Tg of polyimides at least about 28.degree. C. higher than the use temperature, that is, the temperature at which the polyimide is actually used or subjected to, when it is incorporated in an article or operating device.
Although many of the polyimides discussed above and prepared by the processes of the prior art have an adequate Tg and oxidative stability, it is always desirable to improve such properties so that laminates, adhesives, composites and other articles made therefrom withstand prolonged exposure to high pressure air or atmospheric conditions at elevated temperatures. The prior art polyimides discussed above do not have adequate thermal and oxidative stability to permit their use in structural applications requiring prolonged exposure to high pressure air or atmospheric conditions at elevated temperatures, such as at about 370.degree. C.